Manufacture of isoprene



United States I MANUFACTURE OF ISOPRENE James F. Dempsey, Claymont, Del., assignor to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey 7 p Application March 11, 1957, Serial No. 645,352 2 Claims. 01. 260-680 This invention relates to the manufacture of isoprene,

, ,"Ihe eflluent from the dehydrogenation reactor then mono-olefins. ,The bojttoms, comprising isoprene, 2-

methyl-2 butene and piperylengis then superfractionated to'yield a high purity isoprene as an overhead product, and piperylene and 2.-meth yl-2 butene as bottoms. An

" other method of separation isto react the C fraction and more particularly to an improved process for the direct conversion of isopentane to isoprene by catalytic dehydrogenation.

Processes for the production of isoprene by catalytic dehydrogenation of isopentane have heretofore been proposed. some piperylene is produced, in addition to the desired isoprene. In such processes, the reaction products are separated into isoprene, piperylene, unreacted isopentane, isopentene and lower boiling reaction products. The

In all such processes of which I have knowledge,

unreacted isopentane, together with the isopentene, is recomponents, since it could not be expected that itlc'ould 1 be converted to isoprene, and, in fact, the thought has been that the presence of piperylene in the recycle would be detrimental, since it would only act as a diluent, thus reducing the amount of-fresh isopentane that could be charged to the reactor, and lowering the quantity of isoprene that could be produced in a given time.

I have now discovered that, contrary to the prior belief, the presence of piperylene in the feed to a process for dehydrogenating isopentane to isoprene has definite advantages. I have found that if all the C hydrocarbons in the dehydrogenation reaction product, with the exception of isoprene, are recycled to the dehydrogenator, along with fresh feed, that an equilibrium reaction is soon arrived at in which there is no net production of piperylene, the selectivity of the reaction as expressed by the equation:

wt. isoprene in produetX 100 Wt. isopentane converted is greatly increased, and surprisingly, there is no significant loss in production of isoprene on an hourly basis, since the inhibition of piperylene formation counterbalances the smaller actual conversion of isopentane.

In the practice of the present invention, an isopentane feed is contacted at a temperature of from about 900 F. to about 1200 F. with a dehydrogenation catalyst such as an oxide of a group VI metal. A particularly desirable catalyst is activated alumina on which is distended from about 2% to about 10% chromia. The dehydrogenation may be carried out at atmospheric pressure, but subatmospheric pressures in the order of 5 to 10 inches of mercury absolute are preferred, since more Selectivity -eificient conversion of isopentane is had at these lower pressures. Space rates (liquid volume of feed/volume of reactor/hr.) from 0.5 to 5 may advantageously be used;

with S0 to form sulfones fro'm the diolefinspresent, followed by separation of the sulfones from the paraflinsand mono-olefin-s, regeneration of the diolefins from the sulfone's by heating, and fractionation of the diolefins to yield isoprene and piperylene. Stillyanother method is to extract C fraction with a s'olutionof aycopper salt such as copper ammonium acetateto complex the diolefins with the copper salt, followed by stearn stripping t o recover diolefins from theu compleit, and fractionation of the diolefins to yiel'd isoprene and piperylene fractions, Since these, and otherrnethods, for separating isoprene from mixtures of other C hydrocarbons are well known to the art and form no part of the present invention, itisbelieved that any detailed discussion of any one of them would be surplusage, and would not in any waycontribute to an understanding of the present invention. Such discussion will therefore be dis ensed with. v

' In any eventynof matter how the isoprene is separated from the other C5 hydrocarbons; all of the C hyd f carbons in the dehydrogenation product with" the excep tion ;of isoprene are recycled to the dehydrogenation reac tor in admixture with fresh isopentane-feed: "It'will 'ibe found that the pi eryieaeeaatnt at the mixed f eed will rise from an initial value'of 1 orfper'cen't, yuan recycle is firststarted, to a value of1from. about=20 to about 30 percent, depending on the severityof thel condition's in the dehydrogenatcsr. -Whe n the piperylene content of the feed reaches-this-value; there 'will no longer be any net' productit'iri ofIpiperyI eneIaIl of'thediolefin formed inthe -d'eiiydrogenationreaction being: desired isoprene; 'jjItwill be understood that as the piperylene content of thefeed rises; the volume of recycl'ealso rises; that 116W 6f l i. ;si irnare n the re or mnst lessenediri or'der'to maintain a constant space rate in the reactor.

In order that those skilled in the art may more fully appreciate the nature of my invention and a method of carrying it out, it will be more specifically described in connection with the accompanying drawing, which is a flow sheet of one form of the invention.

In starting up operation in accordance with the invention, an isopentane feed stock is taken through line 1 and is passed to furnace 2 in which it is heatedto a temperature of 1100 F. The feed is then passed through line 3 to dehydrogenator 4, which is packed with a catalyst consisting of chromia deposited on activated alumina. The space rate, as defined above, is 1.4. Pressure in dehydrogenator 4 is maintained at 7 inches of mercury absolute by means of eductor 5 located in exit line 6. Eflluent from dehydrogenator 6 is passed to fractionating tower 7, from which hydrocarbons and other products lighter than C are taken overhead through line 8 for further processing. A bottoms product comprising C paraflins, olefins, and diolefins is removed from fractionator 7 through line 9 and is passed to diolefin separation zone 10, in which the diolefins are separated from unreacted isopentane and mono-olefins formed in dehydrogenator 4 in any manner known to the art. These latter compounds are removed from diolefin separation zone 10 via line 11, and are recycled to admixture with feed to dehydrogenator 4 will build up under the reaction conditions given above until it reaches about 28 percent by weight, after which it will r'e'main constant. During this time, the amount'of recycle will gradually build up, and it will be necessary to gradually jdCCICHSE'thC amount of' fresh isopentane introduced "through line 1 in .order to maintain the space velocity in dehydrogenator 4' 'at 'a constant value. 'When' the "process hasgreached equilibrium,the flow of fiesh isopentanethrou'gh line v1 will be about one-fifth of the original volume. I In order todeinonstrate the superior resultsobtained by proceeding "according to the present invention over those achieved by proceeding according to the prior art, comparative data is given in the following table. In the table, case A is an equilibrium process accordingto the prior art, that is, only paraflins and mono-olefins in the reaction product are recycled. Case B is a process according to the present invention, inwhich piperylene is recycled in addition to parafiins and mono-olefins.

,Table I Case A Case B Catalyst Chromia on Same.

Alumina. Space R 1 4 Do. Temperature 1058 11---- Do. Pr r 7 in. Hg Do.

. Absolute. FeedComposition, Wt. Percent:

Isflnenfnne 72.7 52.6.

. Normal 0 1.5- 1.1.

Isopen n 25.8- 18.6. Piperylpnn Nmm 27 7 Product Composition, Wt. Percent Feed:

Hg- 2 4 1.7. 0 -04 and ll-ci- 7-4- 5.4. Isopnnfrma 45 2 32,6, Isopentenes 25.8- 18.6.

' Isopren 1 11.9. Plperylen 4.5. 27.7. C 2.7 2.1.

Selectivity 6 59.5.

As-may be observed from the foregoing, in case A 4.5% of the feed was converted to the undesired piperylene, whereas in case B there is no net production of piperylene. In case A only 43.6% of the isopentane converted went to isoprene, whereas in case B 59.5% of the isopentane reacted was converted to isoprene. In case A 27.5% of the feed was make-up isopentane, and in case B but 20% of the feed was make-up isopentane, yet the production of isoprene was essentially the same in each case. I

The invention claimed is:

1. A process for the production of isoprene which includes passing a feed stock comprising isopentane and from about 20% to about 30% piperylene to a'dehydrogenation zone, contacting it therein with a dehydrogenation catalyst at a temperature of from about 900 F. to about 1200 F., recovering a reaction product, separating the reaction product into isoprene, other five carbon hydrocarbons including piperylene, and lower boiling materials, and recycling all the'five carbon hydrocarbons, with the exception of isoprene, to the dehydrogenation zone in admixture with fresh isopentane in an amount suflicient to replace that converted in the dehydrogenation zone.

2. In the catalytic pyrolysis of isopentane for the production of isoprene, the improvement which comprises effecting the pyrolysis in the presence of from about 20% to about 30%, based on the feed to the process, of added piperylene.

References Cited in thefile of this patent UNITED STATES PATENTS Seyfried et al." June 11, 1946 Sieg Apr. 15, 1958 OTHER REFERENCES. 

1. A PROCESS FOR THE PRODUCTION OF ISOPRENE WHICH INCLUDING PASSING A FEED STOCK COMPRISINGG ISOPENTANE AND FROM ABOUT 20% TO ABOUT 30% PIPERYLENE TO A DEHYDROGENATION ZONE, CONTACTING IT THEREIN WITH A DEHYDROGENATION CATALSYT AT A TEMPERATURE OF FROM ABOUT 900* F. TO ABOUT 1200* F., RECOVERING A REACTION PROUCT, SEPARATING THE REACTION PRODUCT INTO ISOPRENE, OTHER FIVE CARBON HYDROCARBONS INCLUDING PIPERYLENE, AND LOWER BOILING MATERALS AND RECYCLING ALL THE FIVE CARBON HYDROCARBONS, WITH THE EXCEPTION OF ISOPRENE, TO THE DEHYDROGENATION ZONE IN ADMIXTURE WITH FRESH ISOPENTANE IN AN AMOUNT SUFFICIENT TO REPLACE THAT COVERTED IN THE DEHYDROGENEATION ZONE. 